The present invention relates to a circuit arrangement for monitoring and controlling the pressure in the auxiliary pressure supply system of a hydraulic motor vehicle brake arrangement furnished with a pressure accumulator and a hydraulic pump driven by electromotive force. The circuit is activated and deactivated responsive to pressure and is provided with a pressure telltale circuit indicating a decrease in the auxiliary pressure below a predetermined pressure threshold lower than the operating pressure range.
Conventional skid-controlled and non-controlled hydraulic brake systems and brake force boosters of motor vehicles are furnished with an auxiliary pressure supply system including a hydraulic reservoir permanently kept at a high pressure level in order that (with the brakes applied) auxiliary energy can immediately be fed to the brake system by way of the high-pressurized hydraulic fluid. With the aid of a hydraulic pump driven by electromotive force duly activated and deactivated by a pressure switch, the pressure level in the hydraulic reservoir is held within predetermined pressure limit values--i.e., within the operating pressure range. In addition, for safety reasons, a pressure telltale circuit is provided responding to a drop in the pressure level below a predetermined threshold, releasing a warning signal. In the event of a skid-controlled brake system, it turns off the control in whole or part. The telltale threshold preferably is below the operating pressure range in order to release a warning signal only if a defective condition prevails rather than if the pressure approaches the lower limit of the operating pressure range critical of an activation of the hydraulic pump.
With the ignition disconnected and the motor vehicle in the parking position, conventionally, the circuit to the driving motor of the hydraulic pump is discontinued. There is no reloading of the accumulator. As a result of unavoidable valve leakages caused by the permeation of the filling gap through the resilient membrane of standard-type accumulators, which, although relatively low, cannot be entirely eliminated, and also as a result of leakage caused by other weak points, the pressure in the reservoir and in the auxiliary pressure supply system, respectively, will decrease and at least during extended periods of motor vehicle standstill, will drop to below the telltale pressure threshold. High temperatures, unfavorable manufacturing tolerances or wear of the valves will accelerate the pressure drop. Optionally, the pressure can be deliberately decreased during standstill of the motor car, with the aid of auxiliary equipment. The driver, after an extended or even after a short-term standstill of the motor car, is forced afer starting of the motor or activating the ignition to allow for an answering interval and to start off only after he has been notified of the operating pressure in the supply system and, hence, after a proper operating condition of the brake system has been reached.
It is, therefore, the object of the present invention to overcome this disadvantage and to permit an immediate takeoff after starting of the motor after an extended motor car standstill or, at least, reduce the answering interval until disappearance of the warning signal to a period of time considered acceptable. A reduction of the leakages through structurally more complex components should be avoided for cost-saving reasons, especially so as such measures at adverse conditions, such as high temperatures and extended answering intervals, would not bring about the desired result.